David Cameron has repeatedly refused to apologise for saying MPs who vote against airstrikes in Syria are siding with "a bunch of terrorist sympathisers".
The prime minister made the comment at Tuesday night's meeting of the 1922 Committee of backbench Conservative MPs.
"You should not be walking through the lobbies with Jeremy Corbyn and a bunch of terrorist sympathisers," the prime minister told the gathering, according to The Guardian.
Anger among opposition MPs over the prime minister's choice of words threatened to capsize his speech during the crunch Commons debate on Wednesday on whether to authorise airstrikes. Nine MPs interrupted the prime minister to request he withdraw the comments.
"Will he apologise for the remarks he made last night against honourable and right honourable friends on this side of the House?" asked Labour MP Caroline Flint.
The prime minister repeatedly declined to withdraw the comments: "I respect people who disagree. I respect the fact that governments of all colours are have had to fight terrorism and that this is a debate about how to fight terrorism, not whether to fight terrorism."
Emily Thornberry, one of the Labour MPs who complained, said the suggestion was "offensive and it is untrue". MPs from the Lib Dems and SNP also stood up against the move. Former Scottish first minister Alex Salmond said the words were "deeply insulting".
Cameron would not apologise but insisted he respected both stances, saying: "There is honour in voting for. There is honour in voting against."
Labour leader Jeremy Corbyn also opened his response to Cameron with a demand for an apology "It must be treated with the utmost seriousness – and respect given to those who make a different judgement about the right course of action to take," he said.
"Which is why the prime minister's attempt to brand those who plan to vote against the government as 'terrorist sympathisers' both demeans the office of the prime minister and undermines the seriousness of the deliberations we are having today."
The debate is expected to continue until 10pm on Wednesday night, when MPs will vote on whether to authorise further military action in the region.